Add the drupal eslint plugin

[RedLucas]

I personally don't think this takes anything away, but it'd be nice to have this in already for Drupal users.

[illepic]

Revisiting this now that the Drupal 8.4 js coding standards are basically AirBnB: https://www.drupal.org/docs/develop/standards/javascript/eslint-settings

Note the core/package.json now including:

...
    "eslint": "3.19.0",
    "eslint-config-airbnb": "14.1.0",
    "eslint-plugin-import": "2.2.0",
    "eslint-plugin-jsx-a11y": "4.0.0",
    "eslint-plugin-react": "6.10.3",
...

Look for this config making it into p2-theme-core.